Optoelectronic device having an array of smart pixels

ABSTRACT

An optoelectronic device includes a substrate ( 6, 7 ) comprising semiconducting material and an array of smart pixels arranged on or in the substrate, each smart pixel comprising at least one layer ( 12 ) of organic light emitting material, and a light permeable electrode ( 13 ) in contact with the organic layer on a side thereof remote from the substrate. The smart pixels may be capable of one or more of a range of functions, including image sensing, processing, communication and display.

BACKGROUND TO THE INVENTION

The present invention relates to an optoelectronic device.

Organic light emitting diodes (OLEDs) comprise certain organic materialswhich are known to emit light under electrical stimulation. Thematerials can be either small molecules or polymer materials (in polymerlight emitting diodes, PLEDs). These materials require differentprocesses for practical manufacture into devices. Small moleculematerials are deposited onto a substrate by vapour deposition whilstpolymers are cast onto a substrate from a solution by spin-coating,printing, doctor blading or a reel-to-reel process. In a typical polymerLED, a polymer layer is deposited, by spin coating, onto indium tinoxide (ITO) coated glass. This is followed by heat treatment to driveoff residual solvent and a reflective metal electrode is then evaporatedonto the top surface of the polymer layer. The ITO, which istransparent, forms the other electrode and the polymer emits lightthrough the ITO coated glass when a voltage is applied between theelectrodes. Current and voltage control of the light emission is known.

Both types of materials and processes have been used to fabricate arrayson a number of different transparent and non-transparent surfaces.Methods known in the art for creating full colour displays includeink-jet printing of polymer solutions and vapour deposition of smallmolecule materials. Other known methods include the use of monochromedisplays fitted with individual absorptive filters or colour changingmedia filters. Whilst both materials appear compatible with photo-resisttechnology, in practice the processing has reduced the efficiency andlifetime of the devices to unacceptable levels. High-resolution colourand monochrome displays have been demonstrated for small molecules bydepositing them into microcavities. In EP-0,774,787, a full colour OLEDarray is fabricated on a CMOS substrate by this method. The drivers forthe diode array are formed in the substrate.

Various types of liquid crystal display have been fabricated oncrystalline silicon (LCOS) and other silicon materials such aspolysilicon on glass. The silicon material provides the active matrixdrive circuitry as well as the substrate. Similarly, a vacuumfluorescent display has been fabricated on crystalline silicon.

The manufacture of arrays of OLEDs on non-transparent substrates such asCMOS or bi-CMOS is hindered by the need to fabricate an (at least semi-)transparent electrode on top of the organic layers to allow lightemission and viewing. Deposition of indium tin oxide directly onto theorganic layers can cause unacceptable deterioration in the deviceperformance. Another consideration is the need to carefully select thechoice of metal electrode material directly in contact with thesubstrate so that it is fully compatible with microelectronicmanufacturing equipment.

An electronic display is, in effect, a pixelated optoelectronic devicein which electronic information is fed on to the display and convertedto optical information on a pixel-by-pixel basis. A smart pixel array(SPA) is an array of optoelectronic pixels (also called cells or units)in which each pixel has the capability to communicate with other pixelsin the same array or another array by electrical and/or optical means.The configuration of the communication (which cells communicated withwhich others by which means and in which direction) is usuallydynamically programmable by means of optical or electrical signals fedto the SPA. Often communication within an array of pixels on the samesubstrate is done by electronic means while communication between pixelsin separate arrays or on separate or remote substrates is done byoptical means.

SPAs have been implemented in the past in technologies such as, forexample, liquid crystal over crystalline silicon, monolithic III/Vsemiconductor, and III/V semiconductor bonded to CMOS silicon byflip-chip technology.

SPAs have been used in fields as diverse as, for example, imageprocessing, telecommunications switching and optoelectronic neuralnetworks.

The optical communication between SPAs is often carried out using lightor electromagnetic radiation of wavelengths other rather than visible.For example, SPAs used in telecommunications systems often use infra-redwavelengths.

SUMMARY OF THE INVENTION

According to the present invention there is provided an optoelectronicdevice including a substrate comprising semiconducting material and anarray of smart pixels arranged on or in the substrate, each smart pixelcomprising at least one layer of organic light emitting material, and an(at least semi-) transparent electrode in contact with the organic layeron a side thereof remote from the substrate.

Preferably, and in particular where the device forms a display, theelectrode comprises an electrically conducting polymer, and preferably,the surface of the substrate is polished or smoothed to produce a flatsurface.

The substrate may consist of amorphous, polycrystalline ormonocrystalline silicon. Alternatively, the substrate may comprise alayer of amorphous, polycrystalline or monocrystalline silicon overlyinga layer of glass or sapphire. Preferably, the polishing or smoothing ofthe substrate is effected prior to the deposition of the electrode, ororganic, materials of each OLED smart pixel. The smart pixels of thearray may be different, similar or identical, or the array may compriseany two or all three of different, similar and identical smart pixels.Pixels in the array which are physically similar or identical may beprogrammed once and for all or dynamically to perform the same ordifferent functions.

The smart pixels are preferably of the same size and may be arranged assquares or rectangles on a Cartesian grid. For example, see thetwo-by-two pixel corner sections of a pair of adjacent smart pixelarrays illustrated in FIG. 6. Other grids, such as hexagonal pixels on ahexagonal grid or ring, and wedge shaped pixels on a polar or radialgrid, are also feasible as embodiments of the invention.

Each smart pixel of the array is capable of carrying out one, some orall of the following tasks:

-   -   Process information electronically within the pixel    -   Store information within the pixel    -   Transmit electrical signals to one or more other pixels in the        same array by means of conducting layers which form part of an        active circuit (see, for instance, conducting layer “A ” in        FIG. 6) or to one or more pixels in one or more other arrays by        a means conventionally used in electric or optical chip to chip        interconnect (such as copper tracks on a PCB, wires in a ribbon        cable or optical fibres) (see, for instance, interconnect “B” in        FIG. 6)    -   Receive electrical signals from one or more other pixels in the        same array by means of conducting layers which form part of an        active circuit (see, for instance, conducting layer “A” in        FIG. 6) or to one or more pixels in one or more other arrays by        a means conventionally used in electric or optical chip to chip        interconnect (such as copper tracks on a PCB, wires in a ribbon        cable or optical fibres) (see, for instance, interconnect “B” in        FIG.6)    -   Transmit optical signals to one or more other pixels in the same        array or another array by means of light waves propagating in        free space or through an optical system (see, for instance,        means “C” and “D” in FIG. 6)    -   In the case of organic light emitters, such as LEDs,        micro-cavity LEDs, laser diodes or organic modulators utilising        absorption shifting mechanisms, convert electrical signals into        optical signals, for off-chip communication    -   Receive optical signals from one or more other pixels in the        same array or another array by means of light waves propagating        in free space or through an optical system (see, for instance,        means “C” and “D” in FIG. 6)

The above processes may be carried out singly, or some or all togetherat the same time, or in sequence, one after the other or in any othercombination.

Optical signals received by a pixel may be arranged to be converted intoelectrical signals by conversion means, for example, one of thefollowing:

-   -   One or more PN junction diodes or PIN diodes or phototransistors        or photoconductors or one or more other photosensitive elements        or some combination of these, within the active substrate,        (each) electrically connected to suitable amplification circuits        within the active circuit.    -   One or more organic photodiodes or phototransistors or other        photosensitive elements formed in a layer above the active        circuit and (each) electrically connected to the input of a        suitable amplification circuit within the active circuit.    -   Some combination of the above semiconductor photosensitive and        organic photosensitive structures each of which is electrically        connected to the input of a suitable amplification circuit        within the active circuit.

Electrical signals within the pixel are preferably arranged to beconverted into optical signals by means of one or more driver circuits,within the active circuit, (each of) whose output is connected to theunderside of an arrangement of one or more organic light emitting diodesin series or parallel or both, formed in a layer above the activecircuit. The top side of the organic light emitting diode or diodes ispreferably connected to an electrode which is common to some or all ofthe organic light emitting diodes in the smart pixel array. This commonelectrode is preferably of metal and in contact with the substrate.Depending on the relative work functions of the metal and transparentelectrodes, either may serve as the anode with the other constitutingthe cathode.

In one embodiment, the smart pixels are configured to form anoptoelectronic communication link on- and off-chip, for microsystemintegration, computer interconnect, datacom or telecom applications. Forexample, the smart pixels can enable chip-to-chip communication torelieve data bottlenecks, enable optical clock distribution tosynchronise systems or allow direct chip access to optical disks oroptical memories where conversion to/from electrical signals isperformed at the smart pixel.

The pixel configuration may be capable of providing data buffering andmultiplexing and demultiplexing functions and handling data protocols.Organic photodiodes or phototransistors may convert optical signals intoelectrical signals for on-chip communication. Parallel communicationlinks can be provided by spatial, temporal or wavelength multiplexing.Wavelength multiplexing can be provided by different pixelspreferentially emitting and/or absorbing different colours of light. Forexample, microcavity structures, possibly in conjunction with doping ofrare-earth metals, can be employed to give narrow-band sources atselected wavelengths. The doping, microstructuring, or voltage appliedcan be selected to varying the wavelength from pixel to pixel asrequired. Also, organic emitting devices, in conjunction with narrowband photoluminescent, colour conversion structures can be employed.

The optoelectronic communication links can be point-to-point, multi-castor broadcast. The links can be static (fixed) or dynamic(reconfigurable). Optical fibres, optical waveguides or free-spacemicrooptics/optics can be used to transfer the light from source todestination. The organic optoelectronic devices can be micro-structuredto ease optical coupling. Passive or active optical waveguide structuresin organic or inorganic materials can be integrated with the smartpixels on the same substrate.

In one embodiment, the smart pixels are configured as an image sensorand/or display. Broadband or narrowband light in the range ofwavelengths from the ultraviolet to infrared, incident upon the smartpixels can be converted to a digital representation and stored at thepixels. Localised image-processing operations, such as imageenhancement, equalisation or data encryption can be performed. The datacan then be output optically in the form of a displayed image, ortransported elsewhere as optical or electrical communication signals.For example, infra red images can be converted for display in visiblecolours.

In one embodiment, the smart pixels are configured as an image sensorand/or printer. The optical signals can be used to transfer images fromstorage on the pixel array to a light sensitive film.

The organic light emitting material is preferably a polymer but mayalternatively be a monomer or a transition metal chelate. Apart from thelight emitting material, organic layers in the pixel elements mayinclude an electron transport material layer, a hole transport materiallayer, a protective cap material layer and a conducting polymer materiallayer.

As well as a conducting polymer, the (at least semi-) transparentelectrode may comprise further layers, e.g. of indium tin oxide (ITO) orother transparent or semi-transparent metal oxides or low or high workfunction metals, or conducting epoxy resin, deposited onto the organiclayer furthest from the substrate. Alternatively, a glass or plasticsheet, coated with ITO, conducting polymer, or at least one of thelayers that constitute the (at least semi-) transparent electrode, maybe bonded to said furthest layer or another layer of this electrode, tocomplete the electrode and serve as a barrier to the ingress of oxygenand water. The surface of the device may be completed by encapsulationwith a further layer of polymer or glass.

The preferred conducting polymer is poly(ethlyendioxythiophene), sold byBayer AG under the trade mark PEDOT. Other molecularly alteredpoly(thiophenes) are also conducting and could be used, as could theemaraldine salt form of polyaniline. To improve the adherence of PEDOTto certain smooth substrates a polymer blend with a non-conductingpolymer, preferably poly (vinyl alcohol) (PVA), can be made. Forexample, a 9 wt % solution of PVA with PEDOT in a 10(PVA):6 volume ratiocan be used. A wide range of molecular weights of PVA can be usedwithout much difference in the resultant film or its conductivity.

In still another embodiment, a metal electrode may consist of aplurality of metal layers, for example a higher work function metal suchas aluminium deposited on the substrate and a lower work function metalsuch as calcium deposited on the higher work function metal. In anotherexample, a further layer of conducting polymer lies on top of a stablemetal such as aluminium. Preferably, the electrode also acts as a mirrorbehind each pixel and is either deposited on, or sunk into, theplanarised surface of the substrate. However, there may alternatively bea light absorbing black layer or reflective structure between eachpixel.

High work function metals that could be used include tungsten, nickel,cobalt, platinum, palladium and their alloys, and possibly niobium,selenium, gold, chromium, tantalum, hafnium, technetium and theiralloys.

The substrate may also provide data drivers, data converters and scandrivers for processing information to address the array of pixels so asto create images.

In a method of making the optoelectronic device according to theinvention, the organic devices can be integrated directly onto thesemiconductor circuit substrate, formed by vacuum deposition, printing,stencilling or spin coating methods, or formed separately andhybridised, using flip-chip or wafer bonding methods. These processescould be low-temperature (<100° C.) and allow full hermeticencapsulation, to maximise device life-time and performance.

In still another embodiment of the method, selective regions of a bottomconducting polymer layer are made non-conducting by exposure to asuitable aqueous solution allowing formation of arrays of conductingpixel pads which serve as the bottom contacts of the pixel electrodes.

BRIEF DESCRIPTION OF THE DRAWINGS

In order that the present invention may be more readily understood,reference will now be made, by way of example only, to the accompanyingdrawings, in which:—

FIG. 1 is a schematic diagram of a smart pixel which can be implementedusing the invention;

FIG. 2 is a schematic cross section of a smart pixel according to anembodiment of the invention;

FIG. 3 is a schematic cross section of a single pixel of a planarisedsubstrate according to an embodiment of the invention (not showing thepolymer LED);

FIG. 4 is a schematic cross section of an alternative substrate, showingthe deposited polymer LED, and

FIG. 5 is a schematic, fragmentary side view of an array of polymerLEDs.

FIG. 6 is a schematic view of adjacent corners of a pair of smart pixelarrays according to the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIG. 1 shows a smart pixel which is capable of receiving optical signalsat an optical Rx 1 and electrical signals at an electrical Rx 2. Thereceived signals are processed in a processor 3 where data canoptionally be stored. Optical signals are transmitted from an optical Tx4 and electrical signals are transmitted from an electrical Tx 5.

Each smart pixel in the array of the invention may be as exemplified inFIG. 2. The pixel comprises, from bottom to top, the following elements:a passive substrate 6, an active substrate 7, active electronic devices8, circuit electrical interconnections 9, pad connections 10 to organicconducting and light emitting/detecting layers, two unspecified series11, 12 of conducting, organic conducting, hole transport, electrontransport and organic light emitting or detecting layers, and atransparent conducting layer 13 forming a transparent electrode. One ofthe series 11 includes at least one light detecting layer whilst theother series 12 includes at least one light emitting layer. Aplanarising dielectric 14 may cover the active substrate 7.

Once the active matrix circuitry has been fabricated in thesemiconductor substrate, for example using CMOS technology, the surfaceof the substrate can be planarised. This planarisation either takesplace as part of the manufacturing process of the integrated circuit oras a subsequent customising step.

As shown in FIG. 3, the planarisation is effected by depositing thedielectric 14, for example a polymeric material, on the surface of theactive substrate 7. A conducting polymer that can be patterned to createareas of insulation can be used instead for this purpose. A metalmirror/electrode 34, which may be of aluminium, for connecting one ofthe series of layers 11 to the appropriate point in the circuit, is thendeposited, the connection to the circuit being established by a metallicconducting via 36. Metallised portions of the CMOS circuit aredesignated 38.

FIG. 4 shows an alternative arrangement in which the electrode/mirror 34is sunk into the dielectric surface, i.e. full planarisation isachieved.

FIG. 4 also shows one way in which the device construction can becompleted. The series of layers 11 is deposited and the display issealed by coating with a glass plate 42 coated on its inner surface witha transparent conducting layer 43 which comprises the conducting polymerand, optionally, ITO.

FIG. 5 shows an alternative device construction including a particularexample of the series of deposited layers. For simplicity, FIG. 5 showsordinary pixels instead of smart pixels, but the example is equallyapplicable to forming smart pixels according to the invention. On thesubstrate 32 there are deposited, in turn, the planarised aluminiumelectrode/mirror 34, optionally an electron or hole transport layer 44,a light emitting polymer 46, and a transparent electrode 48. Thetransparent electrode may for example consist of a thin layer of highwork function metal 49, of a thickness to be adequately transparent, alayer of conducting polymer 50 and a layer of ITO 51. An encapsulationlayer/barrier 52 which seals all of the LEDs of the array, includingtheir sides, completes this example of the display construction, threepixels of which are shown in FIG. 5.

In manufacturing the display shown in FIG. 5, the flat metal mirrors 34are applied to the surface of the substrate 32 (preferably a CMOS orbi-CMOS backplane) so as to cover most of the area of each pixel withminimal gaps between the mirrors. Chemical Mechanical Polishing may beused to enhance the global and local planarisation.

The layers of polymer and related materials can be deposited by anautomated technique using equipment currently used for applyingphoto-resists used for the patterning of integrated circuit layers. Thisgives precise control and a uniform thickness for each layer.Alternatively, the polymer layers could be ink-jet printed. Rare earthchelates can be vacuum deposited.

The encapsulation layer 52 is applied after making the connections tothe transparent electrode in each pixel. Encapsulation, and possibly theassembly of the pixel, are carried out in clean, dry conditions under apartial vacuum, or a suitable inert or controlled atmosphere.

The display of the invention is robust, the organic LEDs being wellprotected, but has simplified manufacture and encapsulation. The powergenerated as heat should be manageable but could be decreased byreducing the current or voltage used to drive each element. If currentrouting problems arise, multiple power supply bond pads can be used onthe silicon chip.

INDUSTRIAL APPLICABILITY

Devices according to the present invention can be used to implement anyof the following:

-   -   Optoelectronic displays    -   Optoelectronic computing systems, including neural networks and        digital parallel networks    -   Optoelectronic interfaces between the electrical/electronic        domain and the optical domain in datacoms/telecoms, optical        backplanes and chip-to-chip (inter and intra) interconnect    -   Optoelectronic cross-connects, switches, buffers, add/drop        multiplexers, for datacoms/telecoms, optical backplanes and        computing interconnects    -   Smart sensors and/or printers for digital photography,        photolithography and material processing    -   Smart sensors and/or displays, with integrated functions such as        pattern recognition, compressed data, image enhancement, smart        resolution, smart gain, smart colour conversion.

All forms of the verb “to comprise” in this specification mean “toconsist of or include”.

1. An optoelectronic device comprising, in order: a substrate (7)providing active circuitry (8), circuit electrical interconnections (9),and pad connections (10); the optoelectronic device further comprising:a first series (11) of conducting, organic conducting, hole transport,electron transport and organic light emitting layers on one of said padconnections (10), a second series (12) of conducting, organicconducting, hole transport, electron transport and light detectinglayers on another one of said pad connections (10), and a transparentconductive layer (13) forming a transparent electrode, so as to providean array of smart pixels, each smart pixel comprising part of saidactive circuitry.
 2. A device according to claim 1, wherein thesubstrate comprises a semiconducting layer.
 3. A device according toclaim 2, wherein the substrate comprises a layer of silicon.
 4. A deviceaccording to claim 3, wherein said layer of silicon overlies a layer ofinsulating material.
 5. A device according to claim 1, wherein saidtransparent electrode comprises a conducting polymer.
 6. A deviceaccording to claim 5, wherein the conducting polymer is a molecularlyaltered poly(thiophene).
 7. A device according to claim 6, wherein theconducting polymer is poly(ethlyendioxythiophene).
 8. A device accordingto claim 7, wherein the poly(ethlyendioxythiophene) is blended with poly(vinyl alcohol).
 9. A device according to claim 1, wherein a surface ofsaid substrate is flat.
 10. A device according to claim 1, wherein saidarray includes smart pixels that process information electronicallywithin the pixel.
 11. A device according to claim 1, wherein said arrayincludes smart pixels that store information within the pixel.
 12. Adevice according to claim 1, further comprising an electrical means viawhich said smart pixels of said array transmit and/or receiveinformation to or from one or more other pixels.
 13. A device accordingto claim 12, wherein said one or more other pixels are in another array.14. A device according to claim 1, further comprising an optical meansvia which said smart pixels of said array transmit and/or receiveinformation to one or more other pixels.
 15. A device according to claim14, wherein said one or more other pixels are in another array.
 16. Adevice according to claim 1, wherein said array includes smart pixelsthat convert electrical signals into optical signals.
 17. A deviceaccording to claim 16, wherein said smart pixels that convert electricalsignals into optical signals are connected to an arrangement of organiclight emitting diodes formed in a layer over the substrate.
 18. A deviceaccording to claim 16, wherein said smart pixels that convert electricalsignals into optical signals are connected to an arrangement of organiclight modulators or amplifiers formed in a layer over the substrate. 19.A device according to claim 1, wherein said array includes smart pixelsthat modify or amplify light under control of electrical signals.
 20. Adevice according to claim 1, wherein at least one of said active circuitparts of said smart pixels provides a function selected from the groupconsisting of data buffering, multiplexing, demultiplexing, and handlingdata protocols.
 21. A device according to claim 1, further comprisingparallel communication links that are provided by one of spatial,temporal and wavelength multiplexing.
 22. A device according to claim21, wherein different of said pixels emit and/or absorb different colorsof light so as to provide wavelength multiplexing.
 23. A deviceaccording to claim 22, comprising microcavity pixel structures providingnarrowband sources at selected wavelengths.
 24. A device according toclaim 23, wherein said structures are doped differently to vary thewavelength from pixel to pixel.
 25. A device according to claim 24,comprising narrowband photoluminescent colour conversion pixelstructures.
 26. A device according to claim 1, further comprisingpoint-to-point optoelectronic communication links.
 27. A deviceaccording to claim 26, wherein said optoelectronic communication linksare reconfigurable.
 28. A device according to claim 26, furthercomprising waveguide structures integrated with said smart pixels onsaid substrate.
 29. A device according to claim 1, wherein said smartpixels are configured as an intelligent image sensor.
 30. A deviceaccording to claim 29, wherein said smart pixels convert light incidentthereon to a digital representation and store said representation.
 31. Adevice according to claim 1, wherein said smart pixels perform localizedimage processing operations including at least one of image enhancement,equalization and data encryption.
 32. A device according to claim 1,wherein said smart pixels are configured as an intelligent imagedisplay.
 33. A device according to claim 1, wherein said organic lightemitting and light detecting layers are made of a polymer.
 34. A deviceaccording to claim 1, wherein said smart pixels include at least onefurther layer selected from a group consisting of a protective capmaterial layer, and a conducting polymer material layer.
 35. A deviceaccording to claim 1, wherein said transparent electrode includes afurther layer on said organic layer of said pixel that is furthest fromsaid substrate, said further layer selected from a group consisting ofan indium tin oxide layer, another light permeable metal layer, anotherlight permeable metal oxide layer, and a conducting epoxy resin layer.36. A device according to claim 1, further comprising at least oneorganic encapsulation layer on a surface of said device.
 37. A deviceaccording to claim 1, further comprising an electrode deposited on saidsubstrate, said electrode being formed from a higher work function,light permeable, conducting material selected from a group consisting ofaluminum, tungsten, nickel, cobalt, platinum, palladium, niobium,selenium, gold, chromium, tantalum, hafnium, technetium and theiralloys, and indium tin oxide.
 38. A device according to claim 37,wherein said electrode is arranged to act as a mirror behind each pixel.39. A device according to claim 1, further comprising a stable metalelectrode having a layer of conducting polymer overlying said stablemetal electrode.
 40. A device according to claim 39, wherein saidelectrode is arranged to act as a mirror behind each pixel.
 41. A deviceaccording to claim 1, wherein said array of smart pixels has conversionmeans for converting optical signals into electrical signals.
 42. Adevice according to claim 41, wherein said conversion means includes atleast one of a PN junction diode, a PIN diode, a phototransistor and aphotoconductor.
 43. A device according to claim 41, wherein saidconversion means is in said active circuitry.
 44. A device according toclaim 41, wherein said conversion means includes at least one organiclayer formed over said substrate.
 45. A device according to claim 1,further comprising multi-cast optoelectronic communication links.
 46. Adevice according to claim 1, further comprising broadcast optoelectroniccommunication links.
 47. A device according to claim 1, wherein saidorganic light emitting and light detecting layers are made of a monomer.48. A device according to claim 1, wherein said organic light emittingand light detecting layers are made of a transition metal chelate.
 49. Adevice according to claim 1, wherein said substrate includes circuitryfor processing information to address said array of smart pixels so asto create images.